1. Field of the Invention
The present invention relates to a voltage detector for detecting each output voltage of a plurality of unit cells connected in series of an in-vehicle high voltage battery.
2. Description of the Related Art
A Hybrid Electric Vehicle (HEV) utilizing an engine and an electric motor becomes widely used. The HEV has a low voltage battery of 12 volts for starting the engine and a high voltage battery for driving the electric motor. The high voltage battery is formed with a plurality of unit cells, connected in series, such as a nickel-metal hydride secondary battery or a lithium secondary battery.
When the high voltage battery is repeatedly charged and discharged, the unit cells each have a different output voltage or a state of charge (SOC). As for the charge and discharge of the high voltage battery, the charge is terminated when the unit cell having the highest SOC (or output voltage) reaches to a predetermined upper limit of SOC, and the discharge is terminated when the unit cell having the lowest SOC (output voltage) reaches to a predetermined lower limit of SOC, for endurance and safety of the high voltage battery. The uneven SOCs of the unit cells thus actually reduce available capacity of the high voltage battery. In other words, an assistance (to assist the engine with the battery when running up-hill) and a regeneration (to regenerate the battery when running down-hill) become not enough so that a dynamic performance or fuel consumption of the HEV is reduced. It is therefore necessary to detect each output voltage of the unit cells to uniform the SOC of each unit cell.
FIG. 11 shows a conventional voltage detector to detect an output voltage of each unit cell of a high voltage battery (refer to JP,2000-88898, A). BL denotes a low voltage battery, for example, one secondary battery.
BH denotes the high voltage battery to drive an electric motor M and is connected to an alternator (not shown) for charge.
The high voltage battery BH is divided into m blocks B1-Bm and each block has n (arbitrary number) unit cells and each unit cell has x (arbitrary number) secondary batteries.
The voltage detector has voltage detector units 11-1m in a high voltage line and a voltage detector control device 30 (low voltage line CPU 30) in a low voltage line. The low voltage line CPU 30 is powered by the low voltage battery BL and controls the voltage detector units 11-1m corresponding to the blocks B1-Bm, respectively. Each voltage detector unit 11-1m is powered only by the respective blocks B1-Bm. The voltage detector units 11-1m have the respective ground levels for the blocks B1-Bm at negative terminals so that withstand voltages of the voltage detector units 11-1m are reduced.
Each voltage detector unit 11-1m includes a switching unit 21 connected with a positive and negative terminals of each unit cell of the corresponding block B1-Bm. Each switching unit 21 is connected to an associated differential amplifier OP. The output voltages detected by the differential amplifiers OP are converted to digital values with analogue-digital converters (A/D). The converted values are inputted to the low voltage line CPU 30 through insulator interfaces IF11-IF1m, such as photo-couplers, and bus lines BL11-BL1m. 
The low voltage line CPU 30 supplies switching control signals as detection instructions to the switching units 21 via the insulator interfaces IF31-IF3m and bus lines BL31-BL3m to connect the positive and negative terminals of the unit cells C11-Cmn with the differential amplifiers OP in order.
The voltage detector units 11-1m each have a power supply circuit 23, which supplies a constant voltage to operate the associated differential amplifier OP and A/D converter 22 with the associated output voltage of the block B1-Bm, and a cutoff switch Sc1 interconnected between a junction of the differential amplifier OP and A/D converter 22, and the power supply circuit 23.
The low voltage line CPU 30 outputs OFF signals to the cutoff switches Sc1 when the voltages are not detected via the bus lines BL21-BL2m and the insulator interfaces IF21-IF2m. When the cutoff switches Sc1 are turned off, the differential amplifiers OP and the A/D converters 22 are powered off from the blocks B1-Bm to save the power.
The conventional voltage detector inputs the output voltage of each unit cell C11-Cmn detected by the associated differential amplifier OP to the low voltage line CPU 30 in order so that the low voltage line CPU 30 requires output terminals of the same number as the number of the blocks B1-Bm for outputting the switching control signals to the respective voltage detector units 11-1m. The conventional voltage detector also requires the bus lines BL11-BL1m to transmit the switching control signals same as the number of the blocks B1-Bm, resulting to an increase of a number of parts and a large size of the low voltage line CPU 30 and causing an increasing manufacturing cost.
The conventional voltage detector directly outputs the OFF signals to the cutoff switches Sc1 of the blocks B1-Bm from the output terminals through the bus lines BL21-BL2m. The low voltage line CPU 30 requires the m output terminals to output the OFF signals to the blocks B1-Bm and the m bus lines BL21-BL2m to transmit the OFF signals.
The conventional voltage detector requires the bus lines BL11-BL1m and BL21-BL2m to send the switching control signals and the OFF signals of the cutoff switches Sc1 and also requires the insulator interfaces IF11-IF1m and IF21-IF2m so that the low voltage line CPU 30 becomes large and increases the cost.